Electrical power is generally distributed as high voltage alternating current (AC) even though many electrically powered devices operate at a substantially constant, relatively low voltage, referred to as direct current (DC) since use of high voltage allows power to be delivered over large distances with low losses over power lines of reduced cross-section and containing less conductive material while use of AC allows the voltage to be reduced to a desired voltage level using simple transformers. Therefore, other than devices designed to operate from AC power or DC power supplied only from a battery, virtually all devices designed to operate from DC power include an AC-DC power converter, often including voltage regulation. Many devices may require DC power at a plurality of different voltages and thus will generally include DC-DC power converters, as well.
To obtain acceptable efficiency, both AC-DC and DC-DC power converters of current design rely on switching to develop desired voltage levels with sufficient accuracy while accommodating potentially large transients in current that may be drawn by a load. Data processing devices and digital logic circuits that are included in various devices as controls therefor also function by switching. Switching circuits, regardless of the purpose they are intended to serve, inherently produce noise as the switches change state and such switching noise may be propagated back to the power source such as a local power distribution system and be coupled to other devices receiving power from the same source. Switching noise generally contains an unpredictable range of frequency components which can include very high frequencies that may have unpredictable effects in any device that it reaches. For example, high frequency components can be capacitively coupled to signal lines in a logic circuit and cause incorrect operation.
Switching noise may also contain common mode (CM) and differential mode (DM) components. While filtering can reduce the magnitude of switching noise, CM noise components appear as currents in the same direction in both the supply and return paths of a circuit. Common mode noise can be easily transmitted through the parasitic capacitance between primary and secondary windings of a transformer. CM noise is a particular problem in power converters that also provide voltage isolation between the power source and load since current in the same direction on both the supply and return paths will cause the powered device to “float” relative to the power source. Therefore, it has been common in some transformer designs to provide shielding between the primary and secondary windings of some transformers intended for critical applications. However, known types of shielding arrangements have not been particularly effective in holding CM noise to acceptable levels and, in any event, such shielding has been difficult to apply to some transformer designs, particularly in transformers suitable for high power density power converters where one or more of the transformer windings is formed of a pattern of conductive material on a printed circuit board (PCB) or other substrate (collectively referred to as PCB windings) that provides support for other power converter components. Common mode noise can also be coupled through other structures such as heat sinks and ground planes where a parasitic capacitance exists between portions of a transformer and such a structure.